1. Field of the Invention
The present invention relates to a profile control method, wherein a profiling machine, having a non-contact type distance sensor, is used for accurately copying an irregular profile; i.e. the cutting tool is guided by the contour of a model. Particularly, the present invention relates to a profile control method, by which the non-contact type distance sensor can be automatically recovered from a disabled condition when the non-contact type distance sensor has lost its detecting function due to sudden change of the contour of the model during the profile control. Thus, the present invention guarantees the stable execution of the profile control.
2. Description of the Related Art
In a conventional profiling machine, at least one non-contact type distance sensor is mounted on a distal end of the tracer head. A semiconductor laser or light emitting diode (LED) is normally used for this kind of non-contact type distance sensor. With this non-contact type distance sensor, a distance between the tip of the tracer head and the model surface is feedback-controlled to a predetermined value.
However, in this kind of non-contact distance sensor such as an optical sensor, if the model surface inclines largely with respect to a table having the model loaded thereon, an angle of an irradiation beam axis (i.e. a measuring beam axis) coming out of the distance sensor is largely inclined from the perpendicular of the model surface. The resolution of such a distance sensor tends to be adversely lowered because the beam spot formed on the model surface becomes a widely elongated elliptic shape rather than a circle. This results in the deterioration in the accuracy of the profile control.
In order to solve this kind of inconvenience or disadvantage, the applicant of the present invention has already proposed a non-contact profile control apparatus, as disclosed in the Unexamined Japanese Patent Application NO. HEI 1-194500 and the Unexamined Japanese Patent Application NO. HEI 1-328777.
In this non-contact profile control apparatus, a tracer head is provided rotatably about a supporting shaft vertically extending above the table; two sets of optical distance sensors are mounted on the tracer head so that their measuring beams always maintain a predetermined angle with respect to the supporting shaft; the signals obtained from these optical distance sensors are used for obtaining a normal vector of the model surface (a very small plane) irradiated by the measuring beam; and the rotational position of the tracer head is controlled to adjust the direction of the measuring beam as close to the perpendicular of the model surface as possible.
According to the non-contact profile control apparatus, as is proposed above, the distance between the distance sensor and the measuring point on the model surface can always be maintained at a predetermined constant value, and the rotational position of the distance sensor is controlled in such a manner that the measuring beam axis of the distance sensor is always directed as close to the perpendicular of the model surface as possible, thereby making it possible for the cutting operation to be performed accurately through the profile control.
However, if the profile of a model surface to be copied changes radically, the distance between the distance sensor and the model surface will also vary so radically that the distance sensor will sometimes become unable to effectively follow such radical change of the profile.
For example, the measuring beam of the optical distance sensor may deviate out of the detection limit of the optical sensor when the measuring beam passes along a stepped portion formed on the model, or the measuring beam axis of the the distance sensor may suddenly come to have such a largely inclined angle with respect to the model surface that the measuring beam becomes almost parallel with the model surface, thereby causing the beam spot formed on the model surface to become a widely elongated elliptic shape to substantially disable the distance-detection function of the distance sensor and adversely affect the profile control and/or in the rotational position control of the trace head.